1. Field of the Invention
The present invention relates to an exposure apparatus which transfers a predetermined mask pattern onto a photosensitive substrate and, in particular, to an exposure apparatus using an ultraviolet light source suitable for manufacturing semiconductors.
2. Related Background Art
Conventionally, an exposure apparatus such as that schematically shown in FIG. 3A has been known for making semiconductors. As shown in FIG. 3A, a luminous flux from a light source 1 such as a mercury arc lamp is converged by an ellipsoidal mirror 2 and then converted into a parallel luminous flux by a collimator lens 3. The parallel luminous flux then passes through a flyeye lens 4 composed of an assembly of lens elements 4a each having a quadrangular cross section as shown in FIG. 3B or 3C, thereby forming a plurality of light source images on its exit side. An aperture stop 5 having a circular opening is disposed at this light source image position. The luminous fluxes from the plurality of light source images are converged by a condenser lens 6 so as to uniformly illuminate a mask M, which is an object to be illuminated, in a superposing manner.
In the illumination optical apparatus thus configured, by means of a projection optical system 7 composed of lenses 71 and 72, a circuit pattern on the mask M is transferred onto a wafer W which is coated with a resist. This wafer W is mounted on a wafer stage WS which is two-dimensionally movable. The exposure apparatus of FIG. 3A performs a so-called step-and-repeat type exposure operation in which, when an exposure operation for one shot area is completed, the wafer stage WS is successively moved in a two-dimensional manner so as to be repositioned for an exposure operation for the next shot area.
Also, in recent years, there has been proposed a scanning exposure technique in which the mask M is irradiated with a rectangular or arc-like luminous flux, while the mask M and the wafer W which are disposed so as to be conjugate with each other with respect to a projection optical system are scanned in a predetermined direction, in order to transfer the circuit pattern of the mask M onto the wafer W under a high throughput.
In recent years, the output wavelength of the exposure light source has been shortened in order to transfer a finer mask pattern onto the wafer surface. For example, in a so-called projection type exposure apparatus which transfers, by way of a projection optical system, a mask pattern onto a wafer coated with a resist, the resolution of the projection optical system can be improved as the output wavelength of the exposure light source becomes shorter.
When a light source which outputs exposure light of a shorter wavelength such as a pulse light source emitting ultraviolet rays is used, since the wavelength of the emitted light is in the ultraviolet region, quartz glass is used in general as an optical glass material (referred to as "glass material" hereinafter) for transmitting the ultraviolet rays therethrough because it can be processed as easily as materials which can transmit visible rays therethrough.
However, light sources emitting ultraviolet rays such as pulse oscillation light sources for ultraviolet rays have a high output. In particular, in an illumination optical system for an exposure apparatus which guides a luminous flux from such a light source to a mask, there is a position where the diameter of the luminous flux becomes small thereby increasing the energy density at this position such that considerable damage may be imparted to the quartz glass of the illumination optical system. As the wavelength of the light source becomes shorter, the energy further increases so as to cause a greater damage to the quartz glass. Accordingly, the durability of the quartz glass may be problematic.
Also, there has been a demand for increasing the number of substrates such as wafer which are processed per hour in the exposure apparatus for improving the throughput thereof. The power of the light source may be raised in order to effectively increase the illuminance on a substrate such as a wafer, however a higher energy density is therefore imparted to the quartz glass placed at a position where the diameter of the luminous flux is minimized. Accordingly, the durability of the quartz glass may become further problematic.